Good traction and resistance to abrasion are primary considerations for tire treads; however, motor vehicle fuel efficiency concerns argue for a minimization in their rolling resistance, which correlates with a reduction in hysteresis and heat build-up during operation of the tire. These considerations are, to a great extent, competing and somewhat contradictory: treads made from compositions designed to provide good road traction usually exhibit increased rolling resistance and vice versa. Tread compositions typically contain one or more elastomers and one or more types of reinforcing materials such as particulate carbon black and silica; see, e.g., The Vanderbilt Rubber Handbook, 13th ed. (1990), pp. 603-04.
Filler(s), polymer(s), and additives typically are chosen so as to provide an acceptable compromise or balance of the desired properties. Ensuring that reinforcing filler(s) are well dispersed throughout the elastomeric material(s) both enhances processability and acts to improve physical properties. Dispersion of filler particles can be improved by increasing their interaction with the elastomer(s) and/or decreasing their interaction with each other. Examples of efforts of this type include high temperature mixing in the presence of selectively reactive promoters, surface oxidation of compounding materials, surface grafting, and chemically modifying the polymer, typically at a terminus thereof.
Terminal chemical modification often occurs by reaction of a living (i.e., anionically initiated) polymer with a functional terminating agent. Some of the numerous examples of this approach include U.S. Pat. Nos. 3,109,871, 4,647,625, 4,677,153, 5,109,907, 6,977,281, etc., as well as references cited therein and later publications citing these patents.
A recent approach has involved functionalization with a compound that includes an aryl group that includes multiple hydroxyl functionalities (see WO 2009/086490), which has the advantage of providing excellent interactivity with a variety of particulate fillers. This approach also has shown evidence of providing vulcanizates that exhibit good traction properties, even in wet conditions. The functionalized polymers at issue there are said to be capable of preparation via emulsion techniques, often involving free radical initiators, and solution techniques. Within the latter category, anionic initiation and coordination catalysis are described as possible propagation techniques.
Certain types of monomers can be difficult or impossible to polymerize by the foregoing propagation mechanisms. One such monomer is isobutylene.
Tire components like inner liners often employ polyisobutylene (PIB) and copolymers of isobutylene and isoprene (IIR), as well as halogenated versions of the latter. Because of the inability to effectively polymerize isobutylene by anionic or coordination techniques, these types of polymers typically are made by cationic addition polymerizations.